ginga123 wrote:what does the scent smell like? old plastic? old ink smell? both?
The closest thing I know to it is photocopy machine toner. Which is what it might be, as I understand that trace lines are laid onto the acetate with a modified xerox machine.
Anyhow, it's magical to open up a cel that's been bagged for a long time and release the scent.
Sadly, the smell dissipates over time, particularly when the cel is stored in a bag with holes or a corner clipped.
Concerning sketches, I'd just look for heavy use. They've all been handled many times, and each set of sketches is made from the previous ones, so layouts are usually pretty ruinous, roughs are crumpled, gengas are creased, and dougas splattered with paint (and sometimes coffee). This kind of progressive-damage-by-use is very hard to fake, even if the faker wants to make it look used.